Why would a man won’t let you go but wants a formal relationship with his ex now current girlfriend?

I don’t know what to tell my friend. I don’t want to hurt her feelings but from my point of view his just sexually attracted to her.

He acts extremely obsessed with her. She blocks him and he calls her from different numbers and reaches her using different methods. Before going back to her current girlfriend he cheated on her with my friend and when she found out she blocked him. She’s in love and aware. He told her “I love having sex with you because it’s not the same with her”
That happened to me long time ago but I don’t want to hurt her feelings by just telling her he’s sexually obsessed because with me they never acted this obsessed. He used to hide her and when she rejected him he immediately show her current girlfriend the ex he always comes back. He wants her as a mistress but she won’t accept that because she wanted a formal relationship but with her ex current girlfriend was willing to be formal
Why would a man won’t let you go but wants a formal relationship with his ex now current girlfriend?
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